1. Field of the Invention
The present invention relates to an information recording medium and, more particularly, to an erasable recording medium in which information is written by use of a light beam such as a laser beam, the written information itself being read also by such a beam.
The present invention also relates to a method of writing information in the information recording medium and reading out the written information therefrom.
2. Description of the Prior Art
An optical disc is a recording medium which is irradiated by a light beam such as a laser beam, in order to write information thereinto, as well as to read the information written there, by means of reflected light. Conventional examples of erasable optical discs are as follows.
The first type of conventional optical disc utilizes a magneto-optical effect and has a thin amorphous film consisting of a rare earth and a transition metal as a recording layer. In an optical disc of this type, a recording layer is irradiated by a laser beam while a bias magnetic field is being applied thereto, so that reversal of magnetization occurs locally in the irradiated portion, thereby enabling a write operation to be performed. In order to read the information, the recording layer is irradiated by a laser beam, and a very slight Kerr rotation of the reflected light is detected. Therefore, the required optical system becomes complex, and since a magnetic field applying means is required, the necessary write/read system becomes complex and expensive.
The second type of conventional optical disc utilizes a phase transition in the recording layer, to perform a write/erase operation. This disc has a thin film containing Chalcogen such as Te or Se as the recording layer. In this type of optical disc, a difference in reflectivity between a written-in portion and a nonwritten-in portion of the recording layer is detected, in order to read the information written therein, thus eliminating the problem associated with the first type of optical disc. For this reason, in addition to using a thin film containing Chalcogen, various proposals and extensive studies have been made and reported, as described below, concerning the second type of optical disc.
(a) Japanese Patent Disclosure (Kokai) No. 60-179952 describes a rewritable optical disc medium including a thin Au-Te alloy film, having a predetermined composition ratio, as a recording layer.
(b) Japanese Patent Disclosure (Kokai) No. 60-179953 describes an optical disc medium including a thin Ag-Te alloy film, having a predetermined composition ratio, as a recording layer.
(c) Japanese Patent Disclosure (Kokai) No. 60-179954 describes an optical disc including a thin Au-M-Sb alloy film, having a predetermined composition ratio (M is Ag, Cu, Pd, Pt, Al, Si, Ge, Ga, Sn, Te, Se, Bi), as a recording layer.
(d) Japanese Patent Disclosure (Kokai) No. 60-177446 describes an optical disc including a thin In-Sb-M alloy film (M is Au, Ag, Cu, Pd, Pt, Al, Si, Ge, Ga, Sn, Te, Se, Bi) as a recording layer.
According to the following references (e), (f), and (g), when alloy films having compositions corresponding to those of the recording layers disclosed in the above references (a), (b), (c), and (d) are rapidly cooled from the liquid state, they present a metastable phase of a simple cubic lattice structure. These descriptions teach the following:
In the recording mediums described in references (a), (b), (c), and (d), the recording layer is melted by light beam irradiation, and is then rapidly cooled to obtain a metastable state of a simple cubic lattice structure, thereby enhancing the reflectivity at the irradiated portion, to enable information to be written therein. On the other hand, when the written information is to be erased, the recording layer is brought into an equilibrium state of mixed phase, i.e., a low reflectivity state.
Other studies include:
(e) H. L. Lou and W. Klement, Jr.: J. Chem. Phys. vol. 36, pp 1870 to 1874 (1962),
(f) B. C. Giessen, U. Wolff, N. J. Grant: Trans. AIME, vol. 242, p. 597 (1968),
(g) C. B. Jordan: J. Chem. Phys. vol. 39, pp 1613 to 1614 (1963)
However, the second type of optical disc, i.e., the recording medium utilizing a phase transition of the recording layer, to perform the information write/erase operation also possesses the following drawbacks.
Specifically, when a thin film containing Chalcogen is used as a recording layer, the problem of toxicity of the Chalcogen arises, wherein reversibility is gradually lost when it is repeatedly heated by light beam irradiation. In addition, composition margin is narrow because the recording layer is a thin multicomponent film. The recording layer is formed by a vacuum evaporation, resulting in poor reproducibility and mass-productibility. Furthermore, in order to increase write speed, the recording medium is crystalline in its initial state and amorphous in its written state. Therefore, in order to actually use the recording layer as a recording medium, the recording layer must first be annealed, and then crystallized (initial crystallization).
On the other hand, the recording mediums described in the references (a) to (d) possess the following drawbacks:
In these recording mediums, a written portion in the recording layer is in the metastable state of a simple cubic lattice structure, while an erased portion therein is in the equilibrium state. This equilibrium state is a mixed phase consisting of a face-centered cubic structure (Au, Ag), a hexagonal structure (Te) wherein atoms are arranged in the form of a chain, and a rohmbic structure (AuTe.sub.2, Ag.sub.2 Te). These three structures differ from the simple cubic lattice structure which is in the metastable state. They also differ from a dense packing structure. Such a significant difference in crystal structure, between the written and erased states, means that diffusion length of atoms is very large when a phase transition occurs during the write/erase operation. Therefore, a considerable length of time is required to write or erase information by use of a light beam, resulting in an abnormal write/erase operation when a disc (i.e., a recording medium) is rotated at high speed.
Generally, the thin films used as recording layers, as described in references (a) to (d), are formed by a practical vacuum evaporation or by use of a sputtering method. However the recording layers formed by these methods are not in the above-mentioned equillibrium state. They are in a metastable state other than the simple cubic lattice structure. The reflectivity of the recording layer in the as-deposited metastable state is lower than that of the simple cubic lattice structure in the metastable state as the written state, but higher than that of the equilibrium state as the erased state. Therefore, as in the case of the thin Chalcogen film layer described above, in order that the recording layer can be used as the recording medium, it must first be annealed, and a step of crystallizing (initial crystallization) or one of obtaining the equilibrium state equal to the erased state (initial equilibrium process) is also required.
Furthermore, since major components of the recording layers described in references (a) to (d) are Te and Sb, problems of toxicity and poor resistance to environment (heat, humidity, oxidation etc.) arise, as in the case of the thin Chalcogen film.